Usually advancing the timing will hurt top end. It will help the bottom end though.
I wouldn't do it unless you know what you're doing. To much advance and you'll have detonation. Detonation can, and will, destroy an engine in short time.
The problem you run into when you start pushing the advance is that with all the noise a boat makes, exhaust, wind, people screaming, etc, you usually don't hear the detonation. But it is there and it is doing it's damage.